Problem: On the first 7 exams of her history class, Tiffany got an average score of 83. What does she need on the next exam to have an overall average of 85?
Solution: Let her score on the next exam be $x$ The sum of all of her scores is then $7 \cdot 83 + x$ The same sum must also be equal to $8 \cdot 85$ Solve: $x = 8 \cdot 85 - 7 \cdot 83 = 99$.